1. Field of the Invention
The present invention relates to an inkjet printer.
2. Description of the Related Art
An inkjet printer having an inkjet head is known. An inkjet head is provided With a pressure chamber, a piezoelectric actuator sealing the pressure chamber, and a nozzle communicated with the pressure chamber. The pressure chamber is filled with ink when the inkjet printer is used, and the piezoelectric actuator applies pressure to the ink within the pressure chamber. When the ink within the pressure chamber is pushed by a deformation of the piezoelectric actuator, ink is discharged from the nozzle. In this way, the inkjet printer will print text and/or image on a printing medium.
A conventional inkjet printer is disclosed in Japanese Patent Application Publication No. 2004-128492.
The piezoelectric actuator is arranged so as to form at least a portion of a wall defining the corresponding pressure chamber and to seal the pressure chamber. Electrodes are arranged on both surfaces of a piezoelectric film of the piezoelectric actuator.
By applying voltage difference between the electrodes, the piezoelectric actuator deforms due to the piezoelectric effect. Because the piezoelectric actuator forms a portion of the wall defining the pressure chamber, the volume of the pressure chamber changes due to the deformation of the piezoelectric actuator. By deforming the piezoelectric actuator so that the volume of the pressure chamber becomes, smaller, pressure can be applied to the ink within the pressure chamber. By applying pressure to the ink within the pressure chamber, the ink will be discharged from the nozzle that communicates with the pressure chamber.
The so-called “fill before fire” method is often used in order to improve ink discharge efficiency by the piezoelectric actuator. In this method, the following operations are performed. According to “fill before fire” method, a predetermined voltage is applied to the piezoelectric actuator prior to ink discharge operation. Due to the predetermined voltage, the piezoelectric actuator is deformed so that the volume of the pressure chamber becomes smaller. When the ink is to be discharged, an ink discharge signal is applied to the piezoelectric actuator. The ink discharge signal is composed of a combination of an advanced voltage change and a subsequent voltage change. In the advanced voltage change, voltage applied to the piezoelectric actuator is changed from the predetermined voltage that has been applied to the piezoelectric actuator to zero voltage. Due to the advanced voltage change, the piezoelectric actuator is freed from the deformed shape, and the volume of the pressure chamber is increased. At the timing when the volume of the pressure chamber is increased, a negative pressure is generated within the pressure chamber. The generated pressure decrease develops a pressure wave within the pressure chamber. The developed pressure wave will propagate from the pressure chamber to the nozzle through an ink passage communicating the pressure chamber and the nozzle, and the pressure wave is reflected at the nozzle toward the pressure chamber. The reflected pressure wave will return to the pressure chamber. Due to reciprocation of the pressure wave, the pressure within the pressure chamber varies cyclically between a negative pressure and a positive pressure.
According to “fill before fire” method, the subsequent voltage change is applied to the piezoelectric actuator while the pressure wave continues. In the subsequent voltage change, voltage applied to the piezoelectric actuator is changed from zero voltage that has been applied to the piezoelectric actuator to the predetermined voltage. Due to the subsequent voltage change, the piezoelectric actuator is deformed again, and the volume of the pressure chamber is decreased. According to “fill before fire” method, the subsequent voltage change is applied when positive pressure is developed within the pressure chamber due to the pressure wave generated by the advanced voltage change. In this method, the deformation of the piezoelectric actuator to decrease the volume of the pressure chamber and to increase the pressure within the pressure chamber occurs at timing when the pressure within the pressure chamber is increased to the positive value due to the pressure wave. Therefore, a large positive pressure is developed within the pressure chamber by the subsequent voltage change, and ink is effectively discharged from the nozzle. High discharge efficiency may be obtained by “fill before fire” method. The time period from the advanced voltage change to the subsequent voltage change is critical to obtain high ink discharge efficiency.
The pressure wave generated by the advanced voltage change reciprocates a certain distance within the ink passage including an ink storage space, the pressure chamber and the nozzle. This distance is usually referred to acoustic length (AL). The pressure within the pressure chamber vibrates with a cycle time having a value that the acoustic length (AL) is divided by a propagating speed of the pressure wave. When the time period from the advanced voltage change to the subsequent voltage change is appropriate, and the subsequent voltage change is applied at timing when the pressure within the pressure chamber is increased to the positive value due to the pressure wave, high ink discharge efficiency can be obtained. When the time period from the advanced voltage change to the subsequent voltage change is inappropriate, and the subsequent voltage change is applied at timing when the pressure within the pressure chamber is decreased to the negative value due to the pressure wave, high ink discharge efficiency can not be obtained, or ink can not be discharged.